In this report we describe a simple method using the vital dye 5- (and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE) to follow splenic graft rejection by flow cytometry. CFSE-labelled spleen cell suspensions were injected intravenously into various recipients and blood samples were taken at different time points to follow the transferred cells. We found that the labelled cells could be readily detected by flow cytometry for up to eleven weeks. The loss of these labelled cells in various transfusion experiments with different major and minor histocompatibility differences followed the rejection kinetics previously described for skin transplants. Thus, this method offers a simple tool to test the histocompatibility of donor cells/grafts with the host in adoptive/transplantation experiments in which donor and host are not completely syngeneic. Furthermore, we developed a method to trace adoptively transferred fluorescent CFSE-labelled cells by light microscopy by converting in situ immunofluorescence staining into immunoenzyme staining.